The present invention relates to the field of linear motion actuators, and in particular, to a linear motion actuator which utilizes the memory effect provided by a shape memory alloy member.
Linear actuators are considered to be devices which, when supplied with electrical power of appropriate voltage and amperage or when operated in some other manner, provide linear motion so as to actuate another device or machine. A solenoid is a well known type of such linear motion actuator.
However, the defects of a solenoid when used as a linear motion actuator include the following:
(a) a solenoid is complicated in its construction, and it is difficult to reduce its cost and size;
(b) a solenoid is not easily designed for being driven with a low voltage electrical power source--typically, a solenoid requires a large and heavy coil;
(c) the stroke to force relationship for a solenoid is very non linear;
(d) it is difficult to design a solenoid to have a long stroke;
(e) in a plunger type solenoid, there is a noise problem because of the movable iron core hitting the fixed iron core, and, although a so called noiseless type solenoid has been developed, it incorporates less iron and thus it has less power;
(f) a solenoid provides low actuating force at the start of the motion of the core thereof, but in fact in many applications the greatest resistance is presented at the start of motion of the member to be actuated; and accordingly the stroke to force relationship for a solenoid is often very unsuited to particular applications.
A type of linear motion actuator that has recently been developed is one incorporating a coil of shape memory alloy wire material. Shape memory alloy has the property that, when a member made of shape memory alloy is supplied with an output of thermal energy and is heated, as for example by the Joule effect which is produced by an electric current passed through the member, the member exerts a force in the direction which will bring its shape nearer to the original shape via a phase transformation (the reversion transformation from the martensite phase to the parent phase), and its shape tends to alter towards an original that it remembers. This force tending towards alteration of the shape of the member can be utilized for driving a driven member in a desired direction, thus performing mechanical work. In the case of a linear motion actuator as described above, the shape memory alloy wire is wound into a coil, and the coil is arranged to remember a shape in which it has a particular length in its longitudinal direction. In the non actuated condition of the actuator, the coil is biased to have a particular different axial length, and then, when the coil is heated by the passage of an electric current through it, it tries to return to the original length, exerting a force in its longitudinal direction.
However, there are several problems with such prior art devices. First, since its shape memory alloy element is required to be taught in such a coiled shape, this involves cost and difficulties in production. Also, with this coil type shape memory actuator, when the coiled shape memory alloy member is heated by an electric current, relatively large temperature gradients are generated in it, and this may result in heat damage to the coil type shape memory member. Further, during actuation by electric current, some parts of the coil type shape memory actuator may in the worst case not be heated to a proper temperature for phase transformation at all. Also, in the case of such prior art coil type shape memory actuators, the shape recovering force which is being used for motion of the actuated member is the force of the restoration of the original shape of the wire of the coil shape which has become twisted by a torsion deformation, and therefore this shape recovering force is not very strong, because in the case of a torsion deformation the amount of deformation of the wire material of the coil spring is maximum only at the surface of the wire, and decreases towards the central portions of the wire. (As is well known in this art, the greater the amount of deformation of the shape memory alloy member, the greater becomes the shape recovering force thereof).